Double pass coaxial cylinder analyzer with retarding spherical grids

ABSTRACT

A charged particle analyzer is provided wherein a retarding grid consisting of two spaced screen members formed of spherical sections having a concentric center located at the surface of the sample being analyzed is utilized to control the entering velocity of the charged particles into a double pass, coaxial cylinder analyzer. In a specific embodiment, the invention is utilized in an electron spectroscopy for chemical analysis (ESCA) apparatus.

United States Patent Palmherg [54] DOUBLE PASS COAXIAL CYLINDER ANALYZERWITH RETARDING SPHERICAL GRIDS [72] Inventor: Paul W. Palmberg, 50 West93rd Street, Minneapolis, Minn. 5 5420 22 Filed: Aug. 27', 1971 [21]Appl. No.: 175,575

[52] US. Cl ..250/49.5 AE, 250/495 PE, 250/419 Me [51] Int. Cl ..II0lj37/26 [58] Field of Search.250/41.9 ME, 49.5 PE, 49.5 AB

[56] References Cited UNITED STATES PATENTS 3,596,091 7/1971 Helmer..250/49.5 AE

3,631,238 12/1971 MacDonald ..250/49.5 AE

3,582,649 6/1971 Taylor ..250/49.5 AB

[ 51 Oct. 17,1972

OTHER PUBLICATIONS High Sensitivity Auger Electron Spectrometer byPalmberg, Applied Physics Letter Oct. 15, 1969 Vol. 15 No. 8.

Primary Examiner-Lawrence James W. Assistant ExaminerHarold A. DixonAttorney-Schroeder, Siegfried, Ryan & Vidas [57] ABSTRACT A chargedparticle analyzer is provided wherein a retarding grid consisting of twospaced screen members formed of spherical sections having a concentriccenter located at the surface of the sample being analyzed is utilizedto control the entering velocity of the charged particles into a doublepass, coaxial cylinder analyzer. In a specific embodiment, the inventionis utilized in an electron spectroscopy for chemical analysis (ESCA)apparatus.

7 Claims, 6 Drawing Figures PATENTEDum 1 1 m2 SHEET 1 BF 2 DOUBLE PASSCOAXIAL CYLINDER ANALYZER WITH RETARDING SPHERICAL GRIDS chemicalanalysis (ESCA).

In ESCA type analyses, a sample to be analyzed is irradiated withgenerally monochromatic soft X-rays.

These may be Al Ka X-rays which have an energy of about 1486 eV. Othersources of activation may be used, such as an electron gun or highenergy photons. The sample, in turn, gives off emitted electrons whichhave an energy range determined by the elements that the sample is madeof. When the energy distribution is determined with sufficiently highresolution, one can also establish the chemical environment which aparticular atom has based upon the energy distribution of the emittedelectrons.

It is toward the production of an apparatus having an improved degree ofresolution andsensitivity that the present invention is directed. Theobjects of the invention are accomplished through a combination of aspecially constructed retarding grid and a double pass cylindricalmirror analyzer.

- IN THE DRAWINGS FIG. 1 is a side elevational view, mostly incross-section, of an analyzer and certain peripheral equipment inaccordance with the invention;

FIG. 2 is a front elevational view of an aperture disc for use in theapparatus of FIG. 1;

FIG. 3 is a front elevational view of a second aperture for use in theapparatus of FIG. 1;

FIG. 4 is a front elevational view of a field termination plate utilizedin the apparatus of FIG. 1;

FIG. 5 is a side elevational view of a grid forming mold; and

FIG. 6 is a side cross-sectional view of the cylinder analyzer of FIG. 1with a schematic illustration of circuitry for use therewith.

The resolution of a cylinder analyzer is determined by the geometry ofthe instrument. This means that (AE)/E is equal to a constant determinedby the geometry of the instrument. If one reduces the energy ofelectrons entering the cylinder analyzer, one can in crease theresolution of the instrument. For example, if the instrument has adesigned resolution of 0.3 percent, this means that for electrons havingan initial energy of 1000 eV, the resolution will be 3 eV. By decreasingthe energy of electrons from an initialenergy of 1000 eV to 100 eV, witha retarding grid in accordance with the invention, one can increase theresolution of the measurement from 3 eV to 0.3 eV. This resolutionproves adequate for ESCA measurements. I have found, however, that it iscritical that the field utilized to retard the electrons being analyzedbe one of high uniformity and radially directed to the source ofelectrons. To provide this highly uniform field, the retarding gridassembly in accordance with my invention is constructed of twoconcentric spherical sections, as will be described hereinbelow.

Ideally, an analysis should be performed on electrons being emitted froma point source on the sample being analyzed. If this ideal was actuallythe case, the cylinder analyzer would perform in an optimum manner, asfocusing of the electrons analyzed would be lessof a problem. However,in virtually every form of analysis utilizing a cylinder analyzer, thisideal condition is not met. In particular, when one utilized X-rays asthe irradiating source, it is extremely difficult to concentrate theX-rays into a region that could be considered a point source. Imaging ofthe source of the electrons in the sample then becomes a problem, andthe resolution of the instrument is decreased. By utilizing a retardinggrid construction in accordance with my invention in' conjunction with adouble pass analyzer, as will be described in greater detail below, myinvention accomplishes its purposes of providing high resolution eventhough it uses a relatively large source. Through use of my invention,one can irradiate a large region of the sample without adverselyaffecting the analysis, as only those X-ray excited electrons within awell-defined re gion are able to pass through both regions of theanalyzer. The double grid increases the virtual source size bydeflecting electrons which do not originate from the ideal point source.Those electrons which are deflected during passage through the gridswill not have proper trajectories to pass through both analyzers.

Turning first to FIG. 1, there is illustrated in primarilycross-sectional view an analyzer in accordance with the presentinvention. The apparatus in accordance with FIG. 1 includes an innercylinder 11 and an outer cylinder 12, which are held in concentricalignment with one another by end ceramic plates 13 and a center ceramicplate 14. Cylinders 11 and 12 are constructed of a non-magnetic metallike copper, as are other metallic portions unless noted otherwise.Surrounding substantially the entire construction is a magneticshielding member 15 which may be mu metal.

Inner cylinder 11 is provided with a series of annular openings l6, l7,l8 and 19 around the periphery thereof. Small regions 20 of the originalcylinder wall are retained to support sections of the cylinderintermediate openings 16, 17, 18 and 19. Openings l6, l7, l8 and 19 aredesirably covered by a fine mesh metallic screen which is desirably ofabout 100 lines per inch and has a transparency of about percent. Thisscreen permits the majority of the electrons or other charged particlesbeing analyzed to pass through the screen while substantiallyeliminating electric field fringing due to the discontinuity of thewalls produced by these openings.

One end of the inner cylinder 11 is partially closed by grids 21 and 22,which will be described in greater detail below. These grids are spheresegments having a common center of symmetry. At the center of symmetryis located the sample 23 (shown schematically) which is to be analyzedby the instrument. That portion of the sample to be analyzed is locatedat the center of symmetry of the grid members 21 and 22, and also online with the axis of cylinders 11 and 12. A source of irradiationenergy, such as an X-ray source, shown schematically as 24, ispositioned to direct X-rays to strike as closely as possible to thesymmetry point on the sample. The irradiation gives rise to emission ofelectrons from the surface layers of the sample, whose energies arefunctions of the composition of the material and of the chemicalenvironment in which the elements are as sociated. Some of the electronsemitted will follow the curved path illustrated by the dotted line 25through the analyzer.

Turning to FIG. 5, there is illustrated a form for shaping of the gridmembers 21 and 22'. Grid members 21 and 22 are formed of fine wirescreen of the same type utilized in covering the openings of holes 16,17, 18 and 19. The fine stainless steel screen is shaped over a blockgenerally designated 26 of a material such as Teflon. The block willhave a radius of curvature corresponding to that desired for theindividual grid. In the instance of grid.21, .the radius of curvaturewill, of course, be larger than that of grid 22. As typical dimensions,grid 21 will have a radius of about 1 inch, while grid 22 will have aradiusof about 0.9 inches. These are suitable dimensional configurationsfor a cylinder analyzer wherein the inner tube has a diameter of about 2inches, and the outer cylinder has an inner diameter of about 4 36inches. For such a system, the overall length would be approximately 11inches.

In the forming of the screens 21 and.22, a shoulder 27 is provided onthe forming block so as to produce a flanged portion on the screen.Suitable die means are provided for pressing the screen member ontoblock 26 and shoulder 27 to produce the final configuration.

Referring again to FIG. 1, it will be seen that grid 22 is fastened bymeans such as welding to a metal plate43 which is in electrical contactwith outer magnetic shielding member 15. Shielding member iselectrically insulated from the balance of the assembly by ceramic 13.The inner grid 21 is in electrical contact with and mounted to a flange28 projecting into the center of cylinder ll.'Grid 21 and grid 22 aremaintained in electrical isolation from one another by means of ceramicdisc 13. I

Spacing elements 13 and 14 are provided to maintain cylinders 1 1 and12in fixed relationship to one another and to provide a special fieldtermination means to prevent field fringing. Each of these discs 13 and14 is desirably formed of someelectrically insulating material, which ispreferably ceramic. Alumina or quartz are particularly suitable for thepurpose. These discs are provided on the inner surface thereof with aconductive coating 29,. having a high resistivity of about 30 megohms.This high resistance coating aids in the reduction of field fringing,which adversely effects the paths of the electrons as they pass throughthe analyzer.

As furtheraid to reducing the field fringing within the device, Iconstruct the ceramic discs 13 and 14 in such a way that the surfacesinterior to the analyzer have a plurality of concentric relatively highconducting annular rings, as is best seen in FIG. 4. In FIG. 4 there isgenerally illustrated a ceramic disc 13 in front elevationalview, whichhas been provided with a series of concentric conductors in annular formand identified 30 through 34,'respectively.-These rings are concentricwith one another and with the axis of the ceramic plate. 13. Rings 30through 34 are desirably formed by metallizing the surface of theceramic discs 13 or 14 and then, by photolithographic masking andetching, removing the intermediate metal between the rings. The ringswill desirably be about 0.005 inch in width and a few microns inthickness. Suitable metals include gold-chromiumalloys that have beenvacuum sputtered onto the surface of the ceramic. The function of theserings is to provide equi-potential regions on the discs so as tominimize the effects caused by regions of resistivity that are notuniform within film 29.

As an alternative to the construction shown in FIG. 1, the spacer 13 atthe sample end may be in the form of a truncated cone rather than a flatwasher shape as shown. The forward edge of cylinder 12 would then beoffset away from the sample, thus allowing a more simple positioning forthe activation sources 24. These features are described in greaterdetailin my copending application filed of even date herewith entitled FIELDTERMINATION PLATE.

It should be noted with regard to FIG. 1 that the intermediate ceramicdisc 14 has been treated on opposite surfaces thereof in a manneranalogous to that described with regard to FIG. 4. It should also beappreciated that the conductive coating 29 is in electrical contact withcylinders 1 1 and 12 at the inner and outer extremities thereof. Thiscan be readily achieved by metallizing the inner and outer edges of theceramic discs 13 and 14.

At a point midway between the ends of inner cylinder 11, I provide anintermediate aperture disc 35. The aperture disc 35 is secured in fixedrelationship to cylinder 11 by a mounting means generally indicated as36.-Disc 35 is shown in front elevational view in FIG. 2

and consists of a thin metallic disc (desirably of molybdenum) of about0.003 inches thickness, having an opening 37 through the center thereofof about 0.036 inches diameter. This narrow aperture 37 acts as a filterto provide essentially a near point source of electrons for the secondstage of the analyzer.

At the opposite end of the analyzerfrom sample 23. is a second set ofaperture members mounted within a suitable mounting means generallyidentified 38. The end apertures 39 and 40 correspond to theconfigurations shown in FIGS. 3 and 2, respectively. Disc 40 issubstantially identical to that of FIG. 2, both in thickness anddimension, while disc 39 is, as shown in FIG. 3, formed of a thinmolybdenum disc of about 0.003 inches thickness, having a plurality ofannular ring segment openings41 that are positioned concentric with, butoff of, the central axis of the disc. The function of such an off-axisaperture in disc 39 is described in the copending application of Bohn etal., Ser. No. 68,983, for AUGER ELECTRON. SPEC- TROSCOPY.

As a final termination, there is provided a tubular member 42, whichfunctions in the invention as the first dynode of a photomultiplier.

It will now be apparent to the reader that the interaction of theconstruction in accordance with the invention will provide an instrumentcapable of precise determination of the energy distribution of X-rayexcited electrons over a wide energy range. The spherical grids 21 and22, due to their configuration and posi' tioning, act to controllablydecrease the energy of the electrons passing therethrough so as toprovide a resolution of measurement down into a range necessary fordetermining the atomic number and chemical state of the atom from whichthe electron originated. By use of the double pass cylindrical tubeanalyzer in accordance with the invention, the source of electrons isredefined as the electrons pass through the intermediate aperture 37.The second stage of the double pass analyzer has, by virtue of theaperture 37, a source at a precisely known distance from the detector42. Thus the energies of the electrons passing through the second stageis precisely defined by the potential between inner and outer cylinders11 and 12. The detector for the electrons is a electronmultiplier, ofwhich element 42 forms the first dynode. The double pass arrangementfurther provides a construction wherein magnetic effects are essentiallyeliminated by the interior position of the entire second stage of thedouble pass analyzer. The absence of magnetic interference furtherprovides a higher degree of precision of the overall instrument.

Turning now to FIG. 6, there is illustrated in cross section theanalyzer in accordance with FIG. 1, with the principal portions thereofbeing shown in essentially schematic form. Electrical connections aremade as shown to the various portions of the analyzer. The outer mumetal shield 15 is connected by a lead 44 to ground and to one side of avoltage programmable power supply. The outer cylinder 12 is electricallyconnected by lead 45 to a first floating power supply 47, while innercylinder 11 is connected by means of lead 46 to the output of a secondfloating power supply 48. It will be noted that grid 21 is at the samepotential as inner cylinder 11, while grid 22 will be at the samepotential as mu metal shield 15, which is at ground. With power supplies47 and 48 joined to cylinders 11 and 12 as shown, it will be appreciatedthat the potential of cylinder 12 will be at -a value equal to the sumof power supplies 47, 48 and 50 while the potential of cylinder 11 willbe at the potential of power supplies 48 and 50 alone.

A digital ramp generator 49 supplies the timing logic both to thevoltage programmable power supply 50 and to the pulse countingelectronics and digital logic unit. It also supplies a signal to an X-Yrecorder as schematically illustrated in the figure. As the receivingelement for the charged particles (electrons) which pass through thedetector, there is an electron multiplier, which is shown as having apower supply, with its output of the multiplier going to a pulsecounting electronic and digital logic unit. A digital to analogconverter is utilized to convert the signal which then passes to the X-Yrecorder as the Y axis signal.

Various commercial units are available for each of the units ofelectronics utilized in FIG. 6. As illustrative of suitable units, thefloating power supplies 47 and 48 may conveniently be Hewlett PackardModels 62098. The voltage programmable power supply 50 may be ModelOPS2000 of the Kepco Company of Flushing, New York. A digital rampgenerator 49 is conveniently a laboratory computer PDP8/E, and a D/Aconvertor available from Digital Equipment Corporation of Maynard,Massachusetts. A conventional X-Y recorder can be utilized such as Model70048 of the Hewlett Packard Company.

In operation, the system of FIG. 6 functions substantially as follows.Sample 23 is irradiated by suitable means so as to emit electrons. Theseelectrons then pass through screens 22 and 21, which aremaintained atground and at some suitable elevated negative potential, respectively. Ascanning potential is applied to grid 21 to slow down the electronspassing between the two grids to a value such that the resolution of theinstrument can detect the necessary differences in electron energyrequired for the' ESCA measurement. The voltage V, applied between theinner cylinder 11 and outer cylinder 12, where 12 is negative withrespect to 11, is

the magnitude of the voltage that determines the pass energy E of thedouble pass analyzer:

where k is a constant and depends upon the diameters of the inner andouter cylinders of the analyzer. The k is fixed for a particularanalyzer geometry. In order to scan the range of energies of theelectrons being emitted by the sample, one changes the potential ofinner cylinder 11 and thus grid 21 to suitably slow down the electrons.A constant differential between the two cylinders is maintained by meansof a floating power supply 47.

Alternatively, one may hold the potential between grids 22 and 21 at aconstant value and scan by changing the potential between cylinders 11and 12. How ever, under such a condition the energy spread, AE, of theinstrument changes and makes measurements more complex.

Floating power supplies 47 and 48 are chosen so that their ratios ofvoltage are such that the energy of the electrons incident on grids 21and 22 and passed by the analyzer is equal to V the voltage of thevoltage programmable power supply. It should be appreciated that powersupply 48 will be positive with respect to power supply 50 so that thescans can be made down to zero energy.

As the electrons emitted from sample 23 pass through the gridarrangement 22-21 and enter the analyzer via opening 16, they aredeflected by the more negative potential of outer cylinder 12 and followthe path generally marked 25. In this first stage of the analyzer, theelectrons are partially resolved and generally are focused on plate 35containing the aperture 37'. Aperture 37 functions to further resolvethe electrons into a nearly point source of definite size and distancefrom the ultimate receiver 42. In the second stage of the analyzer, theelectrons which pass through opening 37 are further resolved andanalyzed by passage through openings 18 and back through 19 and areultimately focused by means of the double aperture system 39 and 40. Theresulting analysis has a high signal to noise ratio and permits fargreater precision of measurements than has been known with instrumentsknown heretofore.

I claim:

1. An analyzer for determining the energy distribution of chargedparticles being emitted from a source, comprisingi a. first and secondmetallic cylinders, the first of said cylinders having an outer diameterless than that of the inner diameter of the second cylinder and beingpositioned interior to the second cylinder, each of said cylindershaving a common axis;

b. disc-shaped aperture means positioned interior to and intermediatethe ends of said first cylinder in a plane generally perpendicular tosaid axis and dividing said analyzer into first and second stages, saiddisc having an opening therethrough at said axis, the opening definingthe source of charged particles for said second stage;

c. said first cylinder defining a plurality of annularly spaced openingsnear each end of each stage of said first cylinder;

d. a retarding grid arrangement including first and second sphericalsection shaped screens positioned adjacent'a first end of said firstcylinder, said screens being spaced from one another and having a commoncenter of symmetry at the source of charged particles and along saidaxis, said screens being in electrical isolation from one another, theysecond of said screens being inelectrical contact with said firstcylinder;

e. means for applying an electrical potential between said first screenand said second screen and for maintaining a predetermined potentialbetween said first and second cylinders; and

f. charged particle detecting means positioned atthe oppositeend of saidfirst cylinder.

2.'An analyzer in accordance with claim 1 wherein said cylinders areheld in spaced relation by .annular electrically insulating spacerspositioned at each end of the cylinders and at a point midway along thelength thereof.

3. An analyzer in accordance with claim 2 wherein 0.1 inches apart.

6. An analyzer in accordancewith claim 4 wherein the screens are formedfrom line wire of about 100 lines per inch and about percenttransparency.

7. An electron spectroscopy chemical analysis system comprising:

a. first and second metallic cylinders, the first of said cylindershaving an outer diameter less than that of Y the inner diameter of'thesecond cylinder and being positioned'interior to the second cylinder,each of said cylinders having a common axis;

. disc-shaped aperture means positionedinterior to and intermediate theends along the length of said first cylinder in a plane perpendicular tosaid axis and dividing said analyzer into first and second stages, saiddisc having an opening at said axis, the opening defining the source ofcharged particles for said second stage;

c. said first cylinder defining a plurality of annularly spaced openingsnear each end of each stage of said first cylinder;

d. a retarding grid arrangement including first and second sphericalsection shaped screens positioned adjacent the first end of said firstcylinder, said screens being spaced from one another and having a commoncenter of symmetry along said axis, said screens being in electricalisolation from one another, the second of said screens being inelectrical contact with said first cylinder;

e. means for positioning a sample to be analyzed so.

that a portion thereof is along said axis and at the center of symmetryof said screens; f. X-ray source means arranged to direct a beam ofX-rays onto said sample to be analyzed;

g. means for applying electrical potential between said first screen andsaid second screen and for maintaining constant predetermined potentialbetween said first and second cylinders; and

h. electron detecting means positioned at the opposite end of said firstcylinder.

Patent No. 3, 699, 33]. Dated October 17, 1972 Inventor (3) Paul W.Palmberg It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

On the title page, following the inventor's name insert:

assignor to Physical Electronics Industries, Inc. Edina, Minnesota,

a corporation of Minnesota Signed and sealed this 15th day of May 1973.

(SEAL) Attest:

EDWARD M.FLET( JHER,JR. 1 ROBERT GOTTSCHALK Attesting Off cer-Commissioner of Patents F ORM PO-105O (10-69) USCOMM-DC 60376-P69 U.5.GOVERNMENT PRINTING OFFICE: 1969 0-366-334

1. An analyzer for determining the energy distribution of chargedparticles being emitted from a source, comprising: a. first and secondmetallic cylinders, the first of said cylinders having an outer diameterless than that of the inner diameter of the second cylinder and beingpositioned interior to the second cylinder, each of said cylindershaving a common axis; b. disc-shaped aperture means positioned interiorto and intermediate the ends of said first cylinder in a plane generallyperpendicular to said axis and dividing said analyzer into first andsecond stages, said disc having an opening therethrough at said axis,the opening defining the source of charged particles for said secondstage; c. said first cylinder defining a plurality of annularly spacedopenings near each end of each stage of said first cylinder; d. aretarding grid arrangement including first and second spherical sectionshaped screens positioned adjacent a first end of said first cylinder,said screens being spaced from one another and having a common center ofsymmetry at the source of charged particles and along said axis, saidscreens being in electrical isolation from one another, the second ofsaid screens being in electrical contact with said first cylinder; e.means for applying an electrical potential between said first screen andsaid second screen and for maintaining a predetermined potential betweensaid first and second cylinders; and f. charged particle detecting meanspositioned at the opposite end of said first cylinder.
 2. An analyzer inaccordance with claim 1 wherein said cylinders are held in spacedrelation by annular electrically insulating spacers positioned at eachend of the cylinders and at a point midway along the length thereof. 3.An analyzer in accordance with claim 2 wherein the surfaces of thespacers facing the interior space intermediate the first and secondcylinders are coated with a high resistance conductive material which isin electrical contact with each of said cylinders.
 4. An analyzer inaccordance with claim 3 wherein the conductive coating includes aplurality of annular relatively high conductivity rings concentric withsaid axis to provide a plurality of equi-potential regions in saidconductive coating.
 5. An analyzer in accordance with claim 1 whereinthe screens forming the retarding grid are spaced about 0.1 inchesapart.
 6. An analyzer in accordance with claim 4 wherein the screens areformed from line wire of about 100 lines per inch and about 80 percenttransparency.
 7. An electron spectroscopy chemical analysis systemcomprising: a. first and second metallic cylinders, the first of saidcylinders having an outer diameter less than that of the inner diameterof the second cylinder and being positioned interior to the secondcylinder, each of said cylinders having a common axis; b. disc-shapedaperture means positioned interior to and intermediate the ends alongthe length of said first cylinder in a plane perpendicular to said axisand dividing said analyzer into first and second stages, said dischaving an opening at said axis, the opening definiNg the source ofcharged particles for said second stage; c. said first cylinder defininga plurality of annularly spaced openings near each end of each stage ofsaid first cylinder; d. a retarding grid arrangement including first andsecond spherical section shaped screens positioned adjacent the firstend of said first cylinder, said screens being spaced from one anotherand having a common center of symmetry along said axis, said screensbeing in electrical isolation from one another, the second of saidscreens being in electrical contact with said first cylinder; e. meansfor positioning a sample to be analyzed so that a portion thereof isalong said axis and at the center of symmetry of said screens; f. X-raysource means arranged to direct a beam of X-rays onto said sample to beanalyzed; g. means for applying electrical potential between said firstscreen and said second screen and for maintaining constant predeterminedpotential between said first and second cylinders; and h. electrondetecting means positioned at the opposite end of said first cylinder.